a) What is meant by the term "alternating current"? [1] b) Draw a graph showing how the alternating voltage varies with time. [2] c) What are the advantages of using alternating current for the…

a) Alternating current (AC) is a type of electric current in which the direction of the flow of electric charges reverses periodically. Unlike direct current (DC), which flows in one direction, AC changes direction at regular intervals, typically many times per second. This reversal is achieved by alternating voltage, which periodically changes polarity. AC is commonly used in power transmission and household electrical systems due to its efficiency and ease of voltage transformation.

---

b) The graph below shows how the alternating voltage varies with time. The voltage $V$ is plotted on the y-axis, and time $t$ is plotted on the x-axis. The graph is sinusoidal, representing the periodic nature of AC.

Key features of the graph:

• The voltage oscillates between a maximum positive value ($+V_0$) and a maximum negative value ($-V_0$).
• The frequency $f$ (e.g., 50 Hz in Nepal) determines how many complete cycles occur per second.
• The period $T$ (time for one complete cycle) is the reciprocal of the frequency: $T=\frac{1}{f}$.

---

c) Advantages of using alternating current (AC) for the transmission of electrical energy:

1. Ease of Voltage Transformation:

   • AC voltage can be easily increased or decreased using transformers. This allows for efficient transmission over long distances at high voltages (reducing energy loss) and safe distribution at lower voltages.
   • Example: Power stations generate AC at high voltage (e.g., 11 kV), which is stepped up to 132 kV or 400 kV for transmission and then stepped down to 230 V for household use.

2. Reduced Energy Loss:

   • High-voltage AC transmission minimizes energy loss due to resistance in transmission lines (loss $P=I^{2}R$). By increasing voltage, the current $I$ is reduced, lowering $I^{2}R$ losses.

3. Efficient Generation:

   • AC generators (alternators) are simpler and more efficient than DC generators for large-scale power production.

4. Compatibility with Modern Devices:

   • Most household and industrial appliances are designed to operate on AC power.

5. Long-Distance Transmission:

   • AC is better suited for long-distance transmission because transformers can efficiently step up/down voltages, making it practical to transmit power over hundreds of kilometers.

6. Synchronization of Power Grids:

   • AC systems can be easily synchronized across power grids, allowing multiple power stations to feed into a single network seamlessly.

7. Lower Cost:

   • AC systems are generally cheaper to install and maintain compared to DC systems for large-scale applications.